The present invention relates to a hybrid drive unit.
In recent years, so-called hybrid vehicles having an engine and a rotating electric machine as drive power sources for the vehicles are gaining attention from the viewpoints of, for example, fuel economy and environmental protection. In such a hybrid vehicle, the rotating electric machine works as a motor that obtains electric power from a battery to generate a driving force. In addition to transmitting the driving force to a drive mechanism side to achieve motor drive, the rotating electric machine serves, at times, to charge the battery, in which case the machine obtains a driving force from an engine to work as a generator. In addition, the rotating electric machine performs a so-called energy regeneration during braking, recovering an extra inertia force that the vehicle has as the electric power. The rotating electric machine may, in some cases, be used for starting the engine.
An inverter controls operations of the rotating electric machine. The type of inverter intended for motors converts a DC power source supplied from the battery into an AC power source (U-phase, V-phase, and W-phase for a three-phase configuration) through a switching action, and supplies the AC power source to each coil. The type of inverter used with generators, on the other hand, converts AC generated in each coil into a DC power source through, for example, the switching action, and supplies the DC power source to the battery. In order, at this time, to keep the hybrid drive unit mounted on the vehicle as compact as possible and keep the distance between the inverter and the rotating electric machine to be controlled thereby as short as possible so as to inhibit electromagnetic noise from being generated, a known hybrid drive unit includes a main body case that accommodates therein the rotating electric machine, and an inverter case that is integrally formed with the main body case and accommodates therein the inverter.
To begin with, the inverter is formed to include electronic components, such as a switching device, a diode, and a control circuit. Entry of moisture into the inverter case could therefore result in the electric circuit being damaged by, for example, a short circuit. To avoid this, the inverter case is often built to be liquid-tight. Despite the liquid-tight structure, however, it is likely that, when the temperature inside the inverter case increases as a result of the switching action of the inverter, air inside the case expands and even a small amount of the air will leak through the case. When the case is thereafter cooled, the air inside the case contracts to reduce pressure, so that moisture will enter into the inverter case through a fine gap therein.
To lessen the effect from expansion and contraction of air, therefore, the inverter case typically includes a breather for reducing a difference in atmospheric pressure between the inside and the outside of the inverter case by providing communication there between. The breather is basically structured to allow only gas to pass there through, and not a solid or liquid. The structure is nonetheless not able to prevent the entry of moisture completely. It is therefore preferable that the breather be disposed, wherever feasible, at a low position that is less likely to be in contact with the moisture.
Japanese Patent Application Publication No. JP-A-08-65945 (page 4, FIGS. 1 and 2) discloses an exemplary case, in which the position of a breather in an inverter case is devised to eliminate the possibility of water entering the inverter case easily. In a motor drive unit disclosed in Japanese Patent Application Publication No. JP-A-08-65945, the breather is disposed on a bottom surface of the inverter case such that an opening portion of the breather to the outside of the case opposes a boss portion formed concentrically with a motor shaft on a side surface of a main body case that accommodates therein a motor. The breather is formed by combining two vertical paths extending vertically and horizontal paths extending horizontally relative to the bottom surface of the inverter case. In other words, in the motor drive unit disclosed in Japanese Patent Application Publication No. JP-A-08-65945, the breather is structured such that moisture can enter the inside only when the moisture passes through complicatedly bent paths that provide communication between the inside and the outside of the inverter case. In addition, an arrangement is made to permit ventilation on the side of the main body case between the inside of the main body case and the outside of the motor drive unit through a small gap, the inverter case, and the breather, the small gap being formed around a metal rod that penetrates through the bottom surface of the inverter case and extends across the inverter case and the main body case.